A flash memory device, which is a nonvolatile semiconductor memory device, has been widely used in digital cameras, computers mobile telecommunication terminals memory cards, and so on. The flash memory device may be largely categorized into two types: a NOR type flash memory device and a NAND type flash memory device. The NOR type flash memory device is suitable for high-speed program and read operations but may not be suitable for high integration density, since contact holes are formed in source and drain regions of each cell transistor. The NAND type flash memory device may be suitable for high integration density since a plurality of cell transistors are connected in series to form a string.
FIG. 1 is a circuit diagram of a conventional NAND type flash memory array 100. Referring to FIG. 1, the memory cell array 100 includes a plurality of cell strings 110 that are connected to bit lines BL0 and BL1, respectively. Each of the cell strings 110 includes a string selection transistor SST, a ground selection transistor GST, and a plurality of memory cells MC0 through MCm that are connected in series between selection transistors GST and SST. The string selection transistor SST includes a drain connected to the bit line BL0 and a gate connected to a string selection line SSL. The ground selection transistor GST includes a source connected to a common source line CSL and a gate connected to a ground selection line GSL. The memory cells MC0 through MCm connected to word lines WL0 through WLm are connected in series between a source of the string selection transistor SST and a drain of the ground selection transistor GST.
A programming operation and a read operation performed on the NAND type flash memory array will be described. First, the memory cells MC0 through MCm of the memory cell array 100 are erased to have a threshold voltage of −1V, for example, before performing the programming operation thereon. Next, the selected memory cell MC1 is programmed to have a high threshold voltage by applying a high program voltage Vpgm, e.g., 18 through 20V, to the word line WL1 of the selected memory cell MC1, in order to program the memory cells MC0 through MCm. The threshold voltages of the other memory cells MC0, and MC2 through MCm that are not selected are constant.
FIG. 2 is a timing diagram of the read operation of the NAND type flash memory array 100 illustrated in FIG. 1. Referring to FIG. 2, in a bit line precharging section, the bit lines BL0 and BL1 are precharged to a precharge voltage, and 0 V is applied to the string selection line SSL, the ground selection line GSL, the common source line CSL, and all the word lines WL0 through WLm. In a reading section, 0V is applied to the word line WL1 of the selected memory cell MC1; and a read voltage Vread, for example, 4V to 5V, that is greater than the threshold voltages of programmed memory cells is applied to the word lines WL0 and WL2 through WLm of the memory cells MC0 and MC2 through MCm that are not selected, the string selection line SSL, and the ground selection line GSL. Thus, whether the memory cell MC1 is an “ON” cell or an “OFF” cell is determined depending on whether current flows through the cell string 110 of the selected memory cell MC1.
However, when the read operation is repeatedly performed, the read disturb characteristics of a memory cell can cause an “ON” cell to be perceived as being “OFF.” That is, during the read operation, electrons may gradually be injected into a floating gate of a memory cell transistor, and thus, an “ON” memory cell may almost become an “OFF” memory cell, when the read voltage Vread is applied to a word line of an “ON” memory cell.
On a cross-section of the integrated cell string 110, as illustrated in FIG. 3, the string selection line SSL, the ground selection line GSL, and the word lines WL0 through WLm that are connected to the gates of the selection transistors SST and GST and the memory cells MC0 through MCm, are formed at predetermined intervals. Also, the memory cells MC0 through MCm have a coupling ratio of a capacitance Ctun determined by a tunneling oxide layer between a semiconductor substrate and floating gate to a capacitance CONO determined by a dielectric layer between the floating gate and a control gate. The coupling ratio Cr is computed by:
                    Cr        =                              C            ONO                                (                                          C                ONO                            +                              C                tun                                      )                                              (        1        )            
Since the selection transistors SST and GST have a different peripheral pattern from those of memory cells, and thus, a different patterning process is used to manufacture the selection transistors SST and GST. The distance d1 between the string selection line SSL and the adjacent word line WLm and between the ground selection line GSL and the adjacent word line WL0 is set to be longer than the distance d2 between the word lines WL0 through WLm, in consideration of the process patterning process.
Thus, the capacitances Ctun (which is determined by the tunneling oxide layer) of the memory cells MC0 and MCm adjacent to the selection transistors SST and GST can be less than the other memory cells MC1 through MCm-1, and thus, the memory cells MC0 and MCm may have a large coupling ratio. Accordingly, the memory cells MC0 and MCm may have degraded read disturbance characteristics, thereby lowering the reliability of the flash memory device.